1. Field of the Invention
The present invention relates to an electron gun for a color picture tube. Particularly this invention relates to an electron gun for a color picture tube that is capable of reinforcing mechanically an electron lens effect formed by electrodes, so that the discord between beam loci at the peripheral portions of a screen can be prohibited.
2. Description of the Prior Art
A simplified view of a general color picture tube is shown in FIG. 1.
Referring to FIG. 1, a color picture tube 1 is provided with a panel 5 on which fluorescent bodies 8 are coated; a funnel 2; an electron gun 4 for radiating electron beams; a neck 9 in which the electron gun 4 is furnished; a deflection yoke 6 for deflecting the electron beams emanating from the electron gun 4; and a shadow mask 7 for guiding the electron beams to the fluorescent bodies corresponding to each primary additive colors red, green, and blue.
FIG. 2 shows a sectional structure of the electron gun 4. As shown, the electron gun 4 is composed of cathodes 10 for radiating thermions; a first grid electrode 11 and a second grid electrode 12 for forming electron beams by controlling the quantity of the thermions and accelerating the thermions; and a focusing electrode 13 and an acceleration electrode 14 for focusing beam spots on the screen by further focusing the electron beams which passed through the grid electrodes 11 and 12.
Among three electron beams, two outer beams pass through an electron lens formed by both the focusing electrode 13 and the acceleration electrode 14. At the same time, the paths of the two beams are bent towards a central beam by an eccentricity 15 of beam-passing apertures on both the two electrodes 13 and 14. Then, the two outer beams coincide with the central beam, when they arrive at the panel 5.
At this time, the three electron beams, passing through microholes on the shadow mask 7, impinge on the fluorescent bodies. The two outer beams impinge on both red-luminant fluorescent bodies and blue-luminant fluorescent bodies, respectively, while the central beam impinges on green-luminant fluorescent bodies, so that the natural color of red, green, and blue can be realized.
The deflection yoke 6 deflects the electron beams to make the beams impinge on necessary dots on the fluorescent screen.
However, there appears a problem that beam spots focused on the screen does not coincide with one another because the distance between the electron gun and the central portion of the screen differs from that between the electron gun and the peripheral portions of the screen.
To avoid the above problem, typically a self-convergence deflection yoke has been used. As shown in FIGS. 3 and 3A, the self-convergence deflection yoke forms a pincushion magnetic field in a horizontal direction, while forming barrel magnetic field in the direction of a first compensation electrode. This electrode is built in the deflection yoke, perpendicularly to the screen.
However, another problem takes place although the self-convergence deflection yoke can be adopted. When electron beams are, with reference to FIG. 4, deflected or focused, they are normally focused in a horizontal direction, but abnormally over-focused in the direction of the first compensation electrode, or in a vertical direction. That is, a true spot is formed in a horizontal direction, but a halo phenomenon occurs in a vertical direction.
To avoid the above halo, it has been suggested that apertures on the electrodes be made to be eccentric such that astigmatism of the beam spot in the central portion of the screen is positive. The astigmatism means the focusing voltage difference between when the electron beams are accurately focused in a horizontal direction and when they are in a vertical direction.
According to this approach, the beam characteristic on the central portion of the screen becomes a little worse, but the halo at the periphery of the screen can be avoided. That is it restrains a halo phenomenon at the periphery of the screen by trading off improving resolution on the central portion of the screen. But this approach does not fit in with a color picture tube requiring high resolution, either. To avoid those problems, the following method has been developed.
FIG. 5 is a view explaining an optical presentation that the characteristic of an electron lens varies with the variations in voltage on an electrode. The voltage on the electrode varies simultaneously with the deflection yoke, as the electron beams are deflected towards the periphery of the screen. FIGS. 6A and 6B are sectional views showing the electron gun on which such an effect occurs.
The focusing electrode 13 (shown in FIG. 2) is divided into a first focusing electrode 116 and a second focusing electrode 117. To the first focusing electrode 116, a uniform voltage is applied regardless of the deflection yoke. To the second focusing electrode 117, the voltage varying in accordance with the deflection yoke is applied.
Vertical partitions 118, which look like a square bracket as in FIGS. 6C and 6D, are welded on the outer two of the three beam-passing apertures which are located in the front of (in the direction of the screen) the first focusing electrode 116. On three beam-passing apertures located in the rear of (in the direction of the cathode) the second focusing electrode 117, horizontal partitions 119, as shown in FIGS. 6E and 6F, are welded.
The electron beams emanating from the cathodes enter the electron lens which is formed with both the second focusing electrode 117 and the acceleration electrode 114, through the first and second grid electrode 111 and 112. Before entering the electron lens, the electron beams are, as shown in FIG. 7, given a convergent force in a horizontal direction and a divergent force in a vertical direction, which the forces are due to an electric field created between the first and second focusing electrodes 116 and 117. This electric field has been created by the voltage on the second focusing electrode 117. The voltage has increased as the electron beams have been deflected.
Although the beams converge in a horizontal direction, the beam spots are focused accurately in a horizontal direction even at the periphery of the screen because the electron lens becomes weaker owing to the voltage on the second focusing electrode 117. The beam spots vertically diverging by the electron lens are also focused accurately in a vertical direction at the periphery of the screen, by working on with the deflection yoke which deflects an electron beam in a vertical direction.
However, an electron gun for a high-resolution picture tube has a problem that three electron beams cannot easily coincide with one another at the periphery of a screen due to variations in the voltage on a second focusing electrode 117. That is to say, a deflection yoke is usually designed to allow three electron beams to coincide accurately at the central portion of a screen. As the voltage being applied to the second focusing electrode 117 is made to vary in accordance with the deflection of the electron beams, an electron lens becomes weaker and results in discord, though slight, between the beam foci at the periphery of the screen. Resolution of a color picture tube will therefore be deteriorated.